The former president of the European Council has been called upon by Theresa May as a potential "influencer" to break the Brexit deadlock, sources have told Sky News.

It is understood that Herman Van Rompuy had a private dinner with Theresa May's deputy, Cabinet Office Minister David Lidington in Brussels on Monday night.

Mr Van Rompuy, who was European Council president between 2009 and 2014, was spotted arriving at the British ambassador's residence just after 7pm.

A source told Sky News that Mr Van Rompuy was invited to attend the dinner because he is seen as an "influencer" with the potential capability to seek out a compromise between the two sides.

Mr Van Rompuy was at the helm in the European Council throughout the Greek financial crisis, which threatened the stability of the euro. Greece was finally bailed out in a compromise deal overseen by Mr Van Rompuy.

Mr Lidington is believed to have joined Mr Barclay and Mr Barnier for coffee after his dinner meeting with Mr Van Rompuy.

Emerging from the talks at 10pm, Mr Barnier said: "We held constructive talks. It's clear from our side that we are not going to re-open the withdrawal agreement but we will continue our discussion in the coming days. That is all."

Downing Street has not commented on Mr Lidington's meeting with Mr Van Rompuy.

But in a statement on the dinner between Mr Barclay and Mr Barnier, a spokesperson said that they discussed "the next steps" in the UK's withdrawal from the EU and explored whether a way through could be found that would be acceptable to the UK parliament and to the European Union.

The spokesperson said: "The meeting was constructive and Mr Barclay and Mr Barnier agreed to further talks in the coming days and that their teams would continue to work in the meantime on finding a way forward."

Image:Michel Barnier's meeting with Steve Barclay was described as 'productive'

The EU side has stressed that the new talks, which represent the first proper engagement between the two sides since before Christmas, do not constitute new negotiations.

However, both sides do now seem to have placed fresh impetus on finding a new compromise. The surprise presence of Mr Van Rompuy demonstrates an openness to trying new avenues for solutions.

With less than 50 days to go until the UK leaves the European Union, the two sides are still unable to agree on a Brexit deal capable of passing a vote in the House of Commons.

The UK has demanded changes to the Irish backstop element of the Withdrawal Agreement, which was signed off by the two sides in December but has since failed to secure the approval of a majority of MPs in Westminster.

The two sides must reach an agreement and formally ratify it in both the European Parliament and Westminster before 29th March.

If they don't, the will UK leave the EU without a deal, severing - overnight on 29th March - 40 years of economic, trade and legal ties.